If a ship is outside your preception radius for it, it "vanishes". This can be bypassed (if it's hostile) through use of the Disarm or Paralyse hacks. These will not reveal the concealed ship directly, however the "electricity" effect that is overlaid on the ship is not concealed, allowing the position of the hostiles to be known immediately, and their identity inferred from their shape.

To fix: make overlay effects such as EMP and cyberattacks also vanish when outside of perception radius.

Untested: whether this occurs with overlay weapons such as the neuroplague launcher.